1. Field of the Invention
The present invention relates to an electroabsorption modulator, and more specifically, to an electroabsorption modulator appropriate for analog optical communication in which an optical waveguide and an spot size converter are integrated with each other to reduce an insertion loss between an optical fiber and an optical modulator and to favorably operate in high input optical power, and a method of manufacturing the same.
2. Discussion of Related Art
In general, an electroabsorption modulator used for modulating signals in digital optical communication serves to adjust an intensity of output light according to an electrical signal input by interrupting an intensity of incident light (referred to as intensity modulation (IM)). Here, the modulated digital signal is simply classified into a signal having a larger intensity than a certain criterion (state ‘1’, off state) and another signal (state ‘0’, on state). In digital communication, an ‘extinction ratio’ is defined as a unit of intensity, with which on and off states can be distinguished. The extinction ratio varies according to the ability to absorb light with an optical modulator. A multi-quantum well optical modulator typically has an extinction ratio of about 20 dB. In addition to the extinction ratio, as the optical intensity increases, an optical signal to noise ratio (OSNR) in the digital communication grows larger, leading to performance improvement of the system. Therefore, there is a critical need for a digital optical modulator to operate in high input optical power with a large extinction ratio.
In analog optical transmission, an output of light intensity matching to an electrical signal having a certain frequency is modulated and transmitted through an optical fiber, and then an electrical signal is recovered from the optical signal. An optical modulator for use in the analog optical transmission is employed as an essential signal source of a radio-over-fiber (ROF) link optical transmission technology that converts an RF signal carrying a digital modulated signal, such as BPSK, QPSK, and QAM, into an optical signal for transmitting through the optical fiber. In this case, a ratio of an RF signal input to the optical modulator to an RF signal recovered by an optical detector is defined as an RF gain. Further, making the RF gain large is very important in the ROF link optical transmission. From the viewpoint of the optical modulator, the RF gain is proportional to the square of output optical power, and to the slope of a transfer function of the modulator. Therefore, not only the analog optical modulator is required to operate in high input optical power but also the slope of its transfer function should be steep.
In the recent digital or analog communication, the operating speed tends to gradually increase. Further, the operating speed of an electroabsorption modulator is inversely proportional to the capacitance of a device. Therefore, the device size of an optical modulator should be reduced in order to acquire an optical modulator having a fast operating speed, which leads to a reduction in the extinction ratio. Consequently, there is a limit in the speed of the electroabsorption modulator that maintains a certain extinction ratio.
To overcome this limit, a traveling wave electroabsorption modulator was introduced. Theoretically, the traveling wave electroabsorption modulator, which is configured such that light is modulated while an electrical signal and an optical signal propagate at the same speed, is not affected by capacitance of the device. Therefore, an optical modulator having a high operating speed while maintaining a large extinction ratio can be manufactured.
The most key factor in the traveling wave electroabsorption modulator is to uniformly distribute electrical signals and optical signals all over the optical waveguide. In a typical multi-quantum well electroabsorption modulator, an optical confinement factor (OCF) is about 20 to 30%. Here, when a voltage is applied, most incident light is absorbed in a front portion of the device and the absorbed light is split into an electron and a hole, which move to p and n electrodes, respectively. In this case, when light having large intensity is incident, a phenomenon occurs that a very large current is generated at the front portion of the device to break down the device. Likewise, a typical optical modulator having a high OCF to increase optical modulation efficiency cannot operate properly when light having large intensity is input, and, from the viewpoint of the traveling wave optical modulator, the electrical signals are distributed along the optical waveguide while the optical signals are absorbed in the front portion, thus resulting in a negligible difference from a lumped device. Therefore, in order to manufacture a traveling wave electroabsorption modulator that operates in incident light having large intensity, it is important to allow the incident light to be absorbed throughout the waveguide, which is directed to design of an epi-structure having a low OCF.
An optical insertion loss of the typical multi-quantum well electroabsorption modulator is about 10 dB. Here, the loss of about 1 dB is resulted from the multi-quantum well absorption layer and the loss of about 9 dB is attributed to an optical coupling loss with an optical fiber. As described above, the intensity of output light power plays a critical role in performance improvement of the communications in digital and analog communications. Therefore, an optical modulator should be designed to reduce an insertion loss and operate in high input light power.
Most insertion loss is caused by the optical coupling loss with the optical fiber, which is generated from inconsistency of an optical mode between the semiconductor optical device and the optical fiber. The optical mode of the semiconductor optical device is an ellipse with a large diameter of 1 μm, while the optical mode of the optical fiber is a circle with a diameter of about 10 μm. Therefore, in optical coupling between the semiconductor optical device and the optical fiber, a very large loss is unavoidable due to the mismatch of the optical mode size. Thus, one method of reducing the optical coupling loss is to make a shape of the optical mode of the semiconductor optical device close to be circular.
Yuichi Tohmor discloses a method of manufacturing a conventional laser diode integrated with an spot size converter in “Reliability of 1300-nm Spot-Size Converter Integrated Laser Diodes for Low-Cost Optical Modules in Access Network”, (Journal of lightwave technology, Vol. 16 No. 7, pp 1302-1307, 1998). An active portion of the laser diode is removed by etching a place where an spot size converter is to be inserted. Next, using a SAG method, a passive optical waveguide is butt coupled to have an arrangement that the thickness of the passive optical waveguide is reduced from a butt coupling interface, resulting in a thickness of within 0.2 μm at the end.
This arrangement has problems in that the epitaxial re-growth is required in the SAG method, and that the SAG method itself applies a stress to a semiconductor crystal. Thus, there is a problem in that the growth conditions are complicated and should be strictly regulated.
J. E. Johnson discloses a device which is integrated with an electroabsorption modulator, an optical amplifier, and an spot size converter, in “Monolithically Integrated Semiconductor Optical Amplifier and Electroabsorption Modulator with Dual-Waveguide Spot-Size Converter Input” (Journal of selected topics in quantum electronic, Vol. 6, No. 1, pp 19-25, 2000. 1). In this arrangement, a double waveguide structure is used in which the thickness of a passive optical waveguide becomes reduced using the SAG method as it goes to the end portion. Further, active layers for the optical amplifier and the optical modulator have a different bandgap from each other, so that each active layer is also grown using the SAG method. In addition, in order to move the optical mode from an active waveguide of the optical amplifier to the passive waveguide, the active waveguide of the optical amplifier is tapered in the lateral direction. However, this method has also drawbacks in that the manufacture process is complicated by the use of the SAG method.
Yuling Zhuan discloses a multi-quantum well electroabsorption modulator in “Peripheral Coupled Waveguide MQW Electroabsorption Modulator for Near Transparency and High Spurious Free Dynamic Range RF (Fiber-Optic-Link, Photonics technology Letter, Vol. 16 No. 9, pp 2033-2035, pp 2004. 9).
The OCF in the absorption layer located at the multi-quantum well should be small to operate in the high input optical power. In this arrangement, the OCF of the absorption layer located at the multi-quantum well is significantly low, i.e., 5% in the arrangement. Therefore, even with very high input optical power, an amount of optical current generated by input light absorption in the absorption layer is substantially very low. On the other hand, by making the size of the optical mode large, optical coupling efficiency with an optical fiber can be increased.
However, this technology is inappropriate to the digital optical communication, because most of the optical modes exist below the absorption layer and thus a sufficient extinction ratio cannot be obtained. Further, although the size of the optical mode is increased, the shape is still elliptical, resulting in an optical coupling loss with the circular optical fiber.
As described, the prior art uses a SAG method or a peripheral coupled waveguide (PCW) arrangement to manufacture a semiconductor optical device that operate in high input optical power with high optical coupling efficiency with the optical fiber.
However, there are drawbacks in that the SAG method has a complicated manufacturing process, strict processing conditions, and is difficult to manufacture, and that the PCW structure cannot have a sufficient extinction ratio.